Area-type symbology readers typically utilize flash optics similar to the flash optics of a conventional camera. In such readers, a target object is illuminated briefly, during which time the reader produces a digital image of light incident upon an optical detector assembly from its object field. If a target object is positioned within the object field, the detector assembly receives light reflected from the region of the target object positioned in the object field. The digital image is a representation of the reflectance of that region of the target object.
If the reader is not aligned with the target object so that a symbology such as a bar code or other coded symbology on the target object is within the object field, the symbology cannot be read effectively. Therefore, a significant consideration in the use of such readers is the positioning of the reader so that the object field of the detector assembly is in alignment with the symbology on the target object to be read.
Because the illuminating light source is typically not activated at times other than during a flash, this light source provides no prior visible indication of the location of the object field on the target object. As such, the user must guess at the proper alignment of the reader with the target object in order to align the object field with the symbology on the target object to be read. Even if the illuminating light source were to be activated, it would provide little help in orienting the reader, as light is emitted from typical illuminating sources across a wide angular range.
The use of a spotter beam to indicate the general position of the object field for a detector array is known from European Patent Application EPA 0524029A2 to Chandler et al. In such a system, the spotter beam simply provides a single spot indicating roughly the center of the object field to aid in the proper alignment of the reader with the symbology.
An additional concern with readers is the positioning of the detector array at a proper distance from the symbology to be read. Detector assemblies typically employ detector arrays and imaging optics and have an object distance at which their operation, in conjunction with any additional optical elements used in the reader, are most effective. Thus, it is desirable to position the reader at a distance from the target object such that the detector assembly is approximately at the desired object distance from the symbology. Once again, in the absence of a visual aid, a user must make a guess as to the appropriate positioning of the reader with respect to the target object.
The device of Chandler does not provide an aid to a user in positioning the reader at the desired object distance. Instead, the distance from the reader to the target object is determined by capturing the image of the spotter beam and performing calculations with a controller. Upon determining the distance between the reader and the target object, the device adjusts the focal length setting of its imaging optics to adjust the effective object distance of the detector assembly. Such a system requires an adjustable optical imaging system. It also utilizes valuable controller capacity to perform distance calculations.
It is also desirable to provide the user of the reader with an indication of the extent of the object field. As used here, the object field is the region of the object plane imaged by the detector assembly. In that manner, the user will know whether or not the symbology to be read is fully within the object field. When the user knows that the reader has its Object field properly aligned with the symbology, the symbology fully within the object field and the detector assembly at the desired object distance from the symbology, the illuminating light source can be activated to generate a flash of light to illuminate the symbology and produce a high quality digital image from the detector assembly.